This invention relates generally to the positioning and alignment of parts in assemblies of parts and more specifically to the use of inertial navigation techniques in such activities. The types of assemblies for which this invention is most suited are those involving interacting and/or interdependent parts which must be positioned and oriented relative to each other in order for the assembly to work properly and where the relative position and orientation of parts is difficult to determine efficiently with acceptable precision using conventional techniques.
An example of an “assembly of parts” for which this invention is particularly suitable is the automobile which will be used in illustrating how the this invention can be used in the positioning and orientation of parts in the manufacturing stage and also in the maintenance of the vehicle during its lifetime.
A vehicle typically consists of a body, chassis, and the components that facilitate the mobility of the vehicle. The chassis provides a strong, rigid platform which supports the body, the drive train, the wheels, and the suspension components. A typical chassis design involves a structural arrangement of interconnected steel beams. In the beginning, vehicle bodies were bolted to the chassis. The more modern approach has been to integrate the body with the frame by welding with the advantage of achieving greater strength and rigidity of the vehicle while at the same time reducing the strength and rigidity demands on the chassis.
A vehicle requires the precise assembly of a large number of individual components which may require the use of specialized jigs and fixtures and precise measuring systems in the assembly process. For those components that can move out of alignment with respect to the chassis, there must be ways of measuring the out-of-alignment condition and making appropriate adjustments.
Wheel alignment is the condition wherein the orientations of the wheels with respect to the chassis are such as to result in proper vehicle handling and minimal tire wear. Wheels can get out of alignment as a result of (1) wear of the steering and suspension components, (2) bent or damaged steering and suspension components, or (3) sagging springs that result in changes in the alignment angles of the wheels.
The orientation of a wheel is referenced to three axes fixed with respect to the chassis of the vehicle. A “longitudinal chassis axis” is an axis that points in the intended direction of motion of a vehicle when the vehicle is moving along a straight line. A “transverse chassis axis” is an axis that is normal to a longitudinal chassis axis and parallel to the surface which supports the vehicle. A “normal chassis axis” is an axis normal to both a longitudinal chassis axis and a transverse chassis axis.
A “chassis plane” is a plane containing a transverse chassis axis and a longitudinal chassis axis. A “transverse normal plane” is a plane containing a transverse chassis axis and a normal chassis axis. A “longitudinal normal plane” is a plane containing a longitudinal chassis axis and a normal chassis axis.
The “steering axis” is defined as the line drawn through the upper and lower steering pivot points. In the case of a short/long arm (SLA) type suspension system, the upper pivot is the upper ball joint and the lower pivot is the lower ball joint. In the case of a MacPherson strut system, the upper pivot is the center of the upper bearing mount and the lower pivot is the lower ball joint.
The “longitudinal steering-axis plane” is the plane containing the steering axis and a longitudinal axis. The angle between the longitudinal steering-axis plane and a longitudinal normal plane is the “steering axis inclination” (SAI), also known as the “king pin inclination” (KPI). The SAI is positive if the upper end of the steering axis tilts toward the chassis.
The “transverse steering-axis plane” is the plane containing the steering axis and a transverse axis. The angle between the transverse steering-axis plane and a transverse normal plane is called the “caster”, the caster being negative if the upper suspension pivot point is closer to the front of the vehicle than the lower suspension pivot point and positive if the reverse is true.
The “longitudinal wheel-axis plane” is the plane containing the wheel axis and a longitudinal axis. The angle between the longitudinal wheel-axis plane (with the wheels pointed straight ahead) and a chassis plane is called the “camber”, the camber being positive if the top of the wheel is tilted away from the chassis and negative if it is tilted toward the chassis. The sum of the camber and the SAI is called the “included angle”.
The “transverse wheel-axis plane” is the plane containing the wheel axis and a normal axis. The angle (in radians) between the transverse wheel-axis plane (with the wheels pointed straight ahead) and a transverse normal plane is called the “toe angle”, the toe angle being positive if the front of the tire is tilted toward the chassis and negative if it is tilted away from the chassis.
The “turning radius”, also called “toe-out on turns” (TOT or TOOT), is the angle between the transverse wheel-axis planes for the front wheels when the front wheels are placed in a full-turn position.
The angle between the normal to the front axle line, the line connecting the front axles, and a longitudinal axis is called the “setback”. Positive setback results when the right front wheel is set back farther than the left. Negative setback results when the situation is reversed.
The point of intersection of the steering axis and the ground is specified by the “offset” and the “caster trail”. The “offset” is the distance between the point of intersection and the intersection of the central wheel plane and the ground. The “caster trail” is the distance between the point of intersection and the transverse normal plane through the center of the wheel.
The “thrust angle” is the angle of the rear wheel planes with respect to a longitudinal chassis axis.
The most frequent alignment procedure performed is the alignment of the front wheels of the vehicle. The front wheels are attached to the chassis via a number of linkages that permit the wheels to move up and down as the vehicle moves over the ground and to be pointed in a particular steering direction.
The position and orientation of a wheel is conveniently defined by a Cartesian coordinate system fixed with respect to the wheel. The origin of the coordinate system is the intersection of the rotational axis of the wheel and the central plane of the wheel. Two of the axes are in the central plane of the wheel and the third coincides with the axis of rotation of the wheel. The position of a wheel is identified with the origin of the wheel coordinate system. The orientation of the wheel is defined as the orientation of the wheel coordinate system.
The front wheel alignment quantities can be determined by measuring the wheel position and orientation at three turning angles, e.g. −20 degrees, 0 degrees (wheel pointed straight ahead), and +20 degrees while maintaining the upper and lower steering pivot points at fixed positions. The three wheel positions are on a circle concentric with the steering axis. The steering axis corresponds to the line normal to the plane of the circle that passes through the center of the circle. The steering axis inclination is obtained by calculating the angle between a normal chassis axis and the projection of the steering axis on a transverse normal chassis plane. The castor is obtained by calculating the angle between a normal chassis axis and the projection of the steering axis on a longitudinal normal chassis plane. The offset is obtained by calculating the distance between the point of intersection of the steering axis and the driving surface and the line of intersection of the central wheel plane with the driving surface. The castor trail is obtained by calculating the distance between the point of intersection of the steering axis and the driving surface and the line of intersection of the transverse normal chassis plane that contains the wheel axis center point and the driving surface.
The measurement of wheel orientation when the wheel is pointed straight ahead (0 degrees turning angle) provides the data necessary to determine the camber and toe angles. The camber is obtained by calculating the angle between a transverse chassis axis and the projection of the wheel axis onto a transverse normal chassis plane. The toe angle is obtained by calculating the angle between a transverse chassis axis and the projection of the wheel axis onto a longitudinal chassis plane.
The measurement of wheel orientations when the wheels are turned to specified angles provides the data necessary to determine turning radius. The procedure is essentially the same as that for determining toe angles.